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Username Post: Vacuum Advance- how is it supposed to work?        (Topic#239321)
dcairns 
Frequent Contributor
Posts: 2036
dcairns
Loc: Orange CA
Reg: 11-07-03
04-04-10 07:45 AM - Post#1892591    

Can somebody explain how vacuum advance is supposed to work and the pluses and minuses of ported vacuum vs manifold vacuum, or not running vacuum advance at all?
I think I know how this all works, but want to get rid of the mental cobwebs

- Dave
1964 Impala 4-door sedan

_________
/ --------------- \
_/ /___________\ \_
/_________|_________\
|OOO ___________ OOO|
\______|====|______/
|_|------------------|_|




 




Mr. Sinister 
Contributor
Posts: 554
Mr. Sinister
Age: 40
Loc: Fair Hill, MD
Reg: 05-18-09
04-04-10 10:08 AM - Post#1892650    
    In response to dcairns

TIMING AND VACUUM ADVANCE 101

The most important concept to understand is that lean mixtures, such as at idle and steady highway cruise, take longer to burn than rich mixtures; idle in particular, as idle mixture is affected by exhaust gas dilution. This requires that lean mixtures have "the fire lit" earlier in the compression cycle (spark timing advanced), allowing more burn time so that peak cylinder pressure is reached just after TDC for peak efficiency and reduced exhaust gas temperature (wasted combustion energy). Rich mixtures, on the other hand, burn faster than lean mixtures, so they need to have "the fire lit" later in the compression cycle (spark timing retarded slightly) so maximum cylinder pressure is still achieved at the same point after TDC as with the lean mixture, for maximum efficiency.

The centrifugal advance system in a distributor advances spark timing purely as a function of engine rpm (irrespective of engine load or operating conditions), with the amount of advance and the rate at which it comes in determined by the weights and springs on top of the autocam mechanism. The amount of advance added by the distributor, combined with initial static timing, is "total timing" (i.e., the 34-36 degrees at high rpm that most SBC's like). Vacuum advance has absolutely nothing to do with total timing or performance, as when the throttle is opened, manifold vacuum drops essentially to zero, and the vacuum advance drops out entirely; it has no part in the "total timing" equation.

At idle, the engine needs additional spark advance in order to fire that lean, diluted mixture earlier in order to develop maximum cylinder pressure at the proper point, so the vacuum advance can (connected to manifold vacuum, not "ported" vacuum - more on that aberration later) is activated by the high manifold vacuum, and adds about 15 degrees of spark advance, on top of the initial static timing setting (i.e., if your static timing is at 10 degrees, at idle it's actually around 25 degrees with the vacuum advance connected). The same thing occurs at steady-state highway cruise; the mixture is lean, takes longer to burn, the load on the engine is low, the manifold vacuum is high, so the vacuum advance is again deployed, and if you had a timing light set up so you could see the balancer as you were going down the highway, you'd see about 50 degrees advance (10 degrees initial, 20-25 degrees from the centrifugal advance, and 15 degrees from the vacuum advance) at steady-state cruise (it only takes about 40 horsepower to cruise at 50mph).

When you accelerate, the mixture is instantly enriched (by the accelerator pump, power valve, etc.), burns faster, doesn't need the additional spark advance, and when the throttle plates open, manifold vacuum drops, and the vacuum advance can returns to zero, retarding the spark timing back to what is provided by the initial static timing plus the centrifugal advance provided by the distributor at that engine rpm; the vacuum advance doesn't come back into play until you back off the gas and manifold vacuum increases again as you return to steady-state cruise, when the mixture again becomes lean.

The key difference is that centrifugal advance (in the distributor autocam via weights and springs) is purely rpm-sensitive; nothing changes it except changes in rpm. Vacuum advance, on the other hand, responds to engine load and rapidly-changing operating conditions, providing the correct degree of spark advance at any point in time based on engine load, to deal with both lean and rich mixture conditions. By today's terms, this was a relatively crude mechanical system, but it did a good job of optimizing engine efficiency, throttle response, fuel economy, and idle cooling, with absolutely ZERO effect on wide-open throttle performance, as vacuum advance is inoperative under wide-open throttle conditions. In modern cars with computerized engine controllers, all those sensors and the controller change both mixture and spark timing 50 to 100 times per second, and we don't even HAVE a distributor any more - it's all electronic.

Now, to the widely-misunderstood manifold-vs.-ported vacuum aberration. After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark", which moved the vacuum pickup orifice in the carburetor venturi from below the throttle plate (where it was exposed to full manifold vacuum at idle) to above the throttle plate, where it saw no manifold vacuum at all at idle. This meant the vacuum advance was inoperative at idle (retarding spark timing from its optimum value), and these applications also had VERY low initial static timing (usually 4 degrees or less, and some actually were set at 2 degrees AFTER TDC). This was done in order to increase exhaust gas temperature (due to "lighting the fire late") to improve the effectiveness of the "afterburning" of hydrocarbons by the air injected into the exhaust manifolds by the A.I.R. system; as a result, these engines ran like crap, and an enormous amount of wasted heat energy was transferred through the exhaust port walls into the coolant, causing them to run hot at idle - cylinder pressure fell off, engine temperatures went up, combustion efficiency went down the drain, and fuel economy went down with it.

If you look at the centrifugal advance calibrations for these "ported spark, late-timed" engines, you'll see that instead of having 20 degrees of advance, they had up to 34 degrees of advance in the distributor, in order to get back to the 34-36 degrees "total timing" at high rpm wide-open throttle to get some of the performance back. The vacuum advance still worked at steady-state highway cruise (lean mixture = low emissions), but it was inoperative at idle, which caused all manner of problems - "ported vacuum" was strictly an early, pre-converter crude emissions strategy, and nothing more.

What about the Harry high-school non-vacuum advance polished billet "whizbang" distributors you see in the Summit and Jeg's catalogs? They're JUNK on a street-driven car, but some people keep buying them because they're "race car" parts, so they must be "good for my car" - they're NOT. "Race cars" run at wide-open throttle, rich mixture, full load, and high rpm all the time, so they don't need a system (vacuum advance) to deal with the full range of driving conditions encountered in street operation. Anyone driving a street-driven car without manifold-connected vacuum advance is sacrificing idle cooling, throttle response, engine efficiency, and fuel economy, probably because they don't understand what vacuum advance is, how it works, and what it's for - there are lots of long-time experienced "mechanics" who don't understand the principles and operation of vacuum advance either, so they're not alone.

Vacuum advance calibrations are different between stock engines and modified engines, especially if you have a lot of cam and have relatively low manifold vacuum at idle. Most stock vacuum advance cans aren’t fully-deployed until they see about 15” Hg. Manifold vacuum, so those cans don’t work very well on a modified engine; with less than 15” Hg. at a rough idle, the stock can will “dither” in and out in response to the rapidly-changing manifold vacuum, constantly varying the amount of vacuum advance, which creates an unstable idle. Modified engines with more cam that generate less than 15” Hg. of vacuum at idle need a vacuum advance can that’s fully-deployed at least 1”, preferably 2” of vacuum less than idle vacuum level so idle advance is solid and stable; the Echlin #VC-1810 advance can (about $10 at NAPA) provides the same amount of advance as the stock can (15 degrees), but is fully-deployed at only 8” of vacuum, so there is no variation in idle timing even with a stout cam.

For peak engine performance, driveability, idle cooling and efficiency in a street-driven car, you need vacuum advance, connected to full manifold vacuum. Absolutely. Positively. Don't ask Summit or Jeg's about it – they don’t understand it, they're on commission, and they want to sell "race car" parts.

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


 
dcairns 
Frequent Contributor
Posts: 2036
dcairns
Loc: Orange CA
Reg: 11-07-03
04-04-10 02:20 PM - Post#1892774    
    In response to Mr. Sinister

Thank you for taking the time to write all that up. That really gets it clear in my head and gets to the point of a suspicion I have about the idle quality on my 383 in my 64 Impala. I will try out that Echlin vacuum advance can (cheap enough to try and see). My car seems to idle at about 15” and I was noticing the other day, when I put my finger on the bottom of the vacuum advance can to feel the rod, that it was dithering back and forth along with the idle, which has an erratic dive every so often. I am sure there are other issues, but removing this form the soup will help me find the others.

- Dave
1964 Impala 4-door sedan

_________
/ --------------- \
_/ /___________\ \_
/_________|_________\
|OOO ___________ OOO|
\______|====|______/
|_|------------------|_|




 
IgnitionMan 
Valued Contributor
Posts: 3193

Reg: 04-15-05
04-04-10 05:05 PM - Post#1892872    
    In response to dcairns

VC1808, or, CarQuest 57-7535 (same) works as well, just restrict the pin travel to .086 for the 383 on ALL vacuum advances, and, FULL MANIFOLD VACUUM SOURCING, NO PORTED VACUUM.



 
Robert H 
Forum Newbie
Posts: 21

Reg: 04-03-10
04-04-10 07:07 PM - Post#1892944    
    In response to Mr. Sinister

Excellent write-up.



 
Mr. Sinister 
Contributor
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Mr. Sinister
Age: 40
Loc: Fair Hill, MD
Reg: 05-18-09
04-05-10 06:41 AM - Post#1893108    
    In response to Robert H

i just keep it saved on my computer, lol.
no problem, hope it helps!

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


 
sz0k30 
"9th Year" Gold Supporting Member
Posts: 360
sz0k30
Loc: Oakland Co., Michigan
Reg: 10-12-08
04-05-10 06:53 AM - Post#1893116    
    In response to Mr. Sinister

Mr Sinister,

You sound a lot smarter than just 32 years old. And you can spell! Nice write up. Thanks. What do you do?

Roman



 
61ragtop 
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Posts: 660

Age: 41
Loc: Langley B.C. Canada
Reg: 07-30-03
04-05-10 11:00 AM - Post#1893256    
    In response to sz0k30

Listen to Mr.Sinister and Ignitionman! I made the change and wow way more responsive, better miliage, and just runs all around way better!!

So limit the advance and hook it up too FULL manifold vacuum and enjoy.....



 
Petroholic 
"7th Year" Silver Supporting Member
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Petroholic
Age: 50
Loc: Moses Lake, Wa.
Reg: 08-12-05
04-05-10 02:27 PM - Post#1893351    
    In response to 61ragtop

So, run the Echlin #VC-1810 advance can and, set limit the mechanical advance to 20 and run full manifold vacuum, is all there is to it?




 
Mr. Sinister 
Contributor
Posts: 554
Mr. Sinister
Age: 40
Loc: Fair Hill, MD
Reg: 05-18-09
04-05-10 02:50 PM - Post#1893368    
    In response to sz0k30

thanks, i think.
i'm a purchasing and facilities manager, but i've been a hot rodder pretty much my whole life. just a hobbyist. i can't take credit for it, guys have been doing this for years, i just listen when the old hot rodders talk. with all the new technology and parts out there, this is one of those things that still works. it's stuff like this that gives you an advantage.
ignition and timing are pretty simple once you understand how they work.

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


 
Mr. Sinister 
Contributor
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Mr. Sinister
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Loc: Fair Hill, MD
Reg: 05-18-09
04-05-10 03:02 PM - Post#1893374    
    In response to Mr. Sinister

petroholic: yep, pretty much it. you only need to back your mechanical advance off enough to prevent pinging, if present, as vacuum advance only applies to part-throttle operation. the old "32-36 degrees at 3000" rule still applies, and the vacuum can won't affect that. all this assumes you're running 93 octane.
you only need the can if you have low idle vacuum, particularly in gear. most stock style hei cans and even the msd cans are designed to work with 15 inches of vacuum at idle, so while running the low vacuum can won't hurt you, you may not need it.

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


 
Mr. Sinister 
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Mr. Sinister
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Loc: Fair Hill, MD
Reg: 05-18-09
04-05-10 04:25 PM - Post#1893425    
    In response to Mr. Sinister

after chatting with grumpyvette, it got my gears turning. so i thought i'd add some more info.

one of the main reasons that you need a little more timing at idle and just off idle is that you are not filling the cylinders. when the engine is at low rpm, the throat of the carb is nearly closed, not letting much air into the cylinders. it's not filling the cylinders with the intake partially or almost fully restricted and thus when it's compressing the mix, there's less to compress, so there are much lower compression pressures. that lightly compressed mixture, inefficient to begin with as far as ideal A/F ratio, will burn more slowly than a highly compressed, mixed, swirled mixture at a more open throttle position. the charge physically can't mix as well as it does at higher rpms, as not only is there less air and fuel to mix, but the velocity of the incoming charge is lower. the lean mixture takes longer to burn, creating more heat in the cylinder, causing pre-ignition or pinging, which is your air/fuel charge being ignited by the hot spot in the cylinder, and not the spark plug, where and when it should be.

for instance, if you light a fire from many places, it will burn faster than if you light it at one. this may help explain why better fuel atomization (and a wider spray pattern in a fuel injected engine, that covers more of the cylinder) will burn faster. there are more air and fuel molecules to mix and ignite, where in lean mixtures, there are less.

check this video out: http://www.youtube.com/watch?v=cy_yaAOKjA8

you will see that the ignition of the charge is focused more on one side, which takes longer to burn the entire mixture because it has to travel to the leaner side of the chamber and ignite the more widely spaced air and fuel particles. had the incoming charge been more uniform and been able to ignite in a wider pattern across the cylinder, it would burn faster. so, less air/fuel mixture to burn, the longer it takes to burn it.

thus, beginning the burn earlier in the process gives the cylinder more time to burn the lean mixture, leading to a more complete burn and better low rpm combustion, which i can personally attest to. to be very unscientific, with less timing at idle, my engine smells very rich, with more timing, the rich smell is gone. it's because the lean charge, which wasn't burning quickly enough at low rpm, is now being combusted more completely. idle mixture has some affect by adding or subtracting fuel to the incoming air, but it's a fine tuning that should be done after your timing is set. this is why everyone will tell you, changing the settings on your carb means nothing if your timing isn't right.

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


Edited by Mr. Sinister on 04-05-10 04:34 PM. Reason for edit: No reason given.

 
NYH1 
Contributor
Posts: 708

Loc: Central New York
Reg: 07-15-09
04-05-10 04:52 PM - Post#1893448    
    In response to Mr. Sinister

Mr. Sinister, great write up. I'm going to be putting my motor back into my Camaro as soon as I get my transmission rebuilt.

I've honestly never understood vacuum advance. My last Camaro was more of a racecar then a street car. I didn't even use the vacuum advance. This one is going to be driven almost daily and I'm definitely going to use the vacuum advance. I want to be able to tune it the best I can.

The information you posted is going to help a lot!





'78 Camaro, mild Vortec head 385 stroker, 9.1 comp. Lunati Voodoo 262/268 hyd. ft. cam, RPM Intake, 650 AVS carb, TH350 Coan 11" 2600 stall, 8.5" 10 bolt. 3:42 posi, 1 5/8" Full length Headers, 2 1/2" Flowmaster Exhaust System.


 
Mr. Sinister 
Contributor
Posts: 554
Mr. Sinister
Age: 40
Loc: Fair Hill, MD
Reg: 05-18-09
04-05-10 04:55 PM - Post#1893450    
    In response to NYH1

glad it's helping folks and good luck with your camaro!!

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


 
IgnitionMan 
Valued Contributor
Posts: 3193

Reg: 04-15-05
04-05-10 05:24 PM - Post#1893473    
    In response to Mr. Sinister

NOPE, one thing not confirmed, LIMIT THE AMOUNT OF PULL PIN TRAVEL ON THE VACUUM ADVANCE, or, there will be way too much vacuum advance timing when it is plugged into full manifold vacuum.

For a 383, 8 crankshaft degrees is all needed to supplement the initial timing, and that is limiting the pull pin to .086 of an inch travel.

If anyone wants pictures of a stop YOU can make at home, and how to measure the stop distance, send me an e/mail. Let me know which type of distributor you have, large cap HEI, or small cap points type, ask for the vacuum advance stop pics, and they will come to you at no charge, no obligation. Use this e/mail address:

info@davessmallbodyheis.com

And, the whole timing and vacuum advance dissertation above, "TIMING AND VACUUM ADVANCE 101", came from a former GM development engineer, another person who also posts here, but did not add to this topic.



Edited by IgnitionMan on 04-05-10 05:27 PM. Reason for edit: No reason given.

 
Mr. Sinister 
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Mr. Sinister
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Loc: Fair Hill, MD
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04-05-10 05:37 PM - Post#1893489    
    In response to IgnitionMan

ignitionman: this applies only to the aftermarket can?
i have an msd pro-billet rtr distributor, and as far as i know, msd does not make a can the fully deploys at lower vacuum, so i have to find a can that is compatible. i have been talking with a guy on another board that is playing with using a counter spring to have the same affect as a low vacuum can, so i'm waiting to see where that goes. otherwise, i'll be using an aftermarket can as well.

what changes would need to be made for a smaller engine? say like a 350 or a 327? the problem i've run into with my stock msd can, is not being able to get the idle down in park/neutral (1100-1200 after adjusting the idle screw, idle mixture has NO effect) and not being able to get it up in gear (hovers around 500, BADLY chugging the engine). apart form idle issues, response and low end power are greatly improved, with a noticeable reduction in that "rich" smell out the tailpipes. the only variable left if the low vacuum can, which i have yet to replace.

and yes, that writeup has been around for quite a while, i don't try to take credit for it.

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


Edited by Mr. Sinister on 04-05-10 05:41 PM. Reason for edit: No reason given.

 
Mr. Sinister 
Contributor
Posts: 554
Mr. Sinister
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Loc: Fair Hill, MD
Reg: 05-18-09
04-05-10 05:57 PM - Post#1893508    
    In response to Mr. Sinister

here's the original discussion i read on the subject. http://www.hotrodders.com/forum/vacuum-advance- hoo...
the article has been posted and reposted many, many times on many different forums and has been proven time and time again. i had not read about ignitionman's pointer about limiting pin travel before, so it's probably worth checking into.

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


 
Trucked_up 
Valued Contributor
Posts: 4948

Loc: 315 er
Reg: 03-23-02
04-06-10 03:06 AM - Post#1893671    
    In response to Mr. Sinister

  • Quote:
After 30-40 years of controlling vacuum advance with full manifold vacuum, along came emissions requirements, years before catalytic converter technology had been developed, and all manner of crude band-aid systems were developed to try and reduce hydrocarbons and oxides of nitrogen in the exhaust stream. One of these band-aids was "ported spark",


Who ever wrote that never worked on older vehicles.many,probably almost all used ported vacuum,cars and trucks.
But the full vacuum does work nice on a hi performance street engine.



 
Mr. Sinister 
Contributor
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Mr. Sinister
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04-06-10 08:17 AM - Post#1893778    
    In response to Trucked_up

yeah, switching to full manifold vacuum probably doesn't do much for a stock engine, it's definitely more for performance applications.

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


 
Mr. Sinister 
Contributor
Posts: 554
Mr. Sinister
Age: 40
Loc: Fair Hill, MD
Reg: 05-18-09
04-06-10 09:21 AM - Post#1893796    
    In response to Mr. Sinister

also, it depends on how old you're talking, my dad's 59 vette is 100% original and has a hollow carb stud that the vacuum line for the advance hooks up to, to draw full manifold vacuum. from my understanding, this was pretty common. in all my reading, ported vacuum became commonplace on emissions era cars.

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


 
Rick_L 
Honored Member
Posts: 27206
Rick_L
Loc: Katy, Tx, USA
Reg: 07-06-00
04-06-10 09:44 AM - Post#1893807    
    In response to Mr. Sinister

"yeah, switching to full manifold vacuum probably doesn't do much for a stock engine, it's definitely more for performance applications."

Trust me, they ALL like full manifold vacuum.

The only reason the ported vacuum was used, along with very little initial timing, was smog engines of the 70s and early 80s needed a lot of heat in the exhaust to make the early catalytic converters work. It wasn't for performance, economy, or driveability, because it hurt all three.

Now that cats have advanced a bit, the carmakers run a "normal" amount of vacuum advance, it's just programmed into the engine computer instead of being analog.



 
steeler_fan 
Contributor
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steeler_fan
Loc: Folsom Ca
Reg: 02-16-07
04-06-10 12:13 PM - Post#1893880    
    In response to Rick_L


This is article from another board. it explains in detail the operation of Vacuum cans and give information on cans availabe. With this you can find the vacuum that a can will engage and the amount of vacuum timing added.


Distributor Vacuum Advance Control units
Specs and facts for GM Distributors

by Lars Grimsrud
SVE Automotive Restoration
Musclecar, Collector & Exotic Auto Repair & Restoration
Broomfield, CO Rev. B 8-19-02


I’ve been seeing a lot of discussion and questions regarding distributor vacuum advance control units; what do they do, which ones are best, what was used on what, etc., etc. To clarify some of this, I thought I’d summarize a few facts and definitions, and provide a complete part number and specification listing for all vacuum advance control units used by Chevrolet on the points-style distributors. I’m also providing a listing of the specs for all other GM (non-Chevrolet) control units, but without the specific application listed for each (it would take me a bit too much time to research each part number by application across each of the GM Motor Divisions – it took me long enough to compile just the Chevy stuff…!). This latest revision to this paper also includes the HEI listings (the HEI distributors use a longer control unit, so the non-HEI and HEI vacuum advance control units CANNOT be interchanged).

As always, I’m going to include the disclaimer that many of these are my own comments and opinions based on my personal tuning experience. Others may have differing opinions & tuning techniques from those presented here. I have made every attempt to present factual, technically accurate data wherever possible. If you find factual errors in this information, please let me know so I can correct it.

Background
The vacuum advance control unit on the distributor is intended to advance the ignition timing above and beyond the limits of the mechanical advance (mechanical advance consists of the initial timing plus the centrifugal advance that the distributor adds as rpm comes up) under light to medium throttle settings. When the load on the engine is light or moderate, the timing can be advanced to improve fuel economy and throttle response. Once the engine load increases, this “over-advance” condition must be eliminated to produce peak power and to eliminate the possibility of detonation (“engine knock”). A control unit that responds to engine vacuum performs this job remarkably well.

Most GM V8 engines (not including “fast-burn” style heads), and specifically Chevys, will produce peak torque and power at wide open throttle with a total timing advance of 36 degrees (some will take 38). Also, a GM V8 engine, under light load and steady-state cruise, will accept a maximum timing advance of about 52 degrees. Some will take up to 54 degrees advance under these conditions. Once you advance the timing beyond this, the engine/car will start to “chug” or “jerk” at cruise due to the over-advanced timing condition. Anything less than 52 degrees produces less than optimum fuel economy at cruise speed.

The additional timing produced by the vacuum advance control unit must be tailored and matched to the engine and the distributor’s mechanical advance curve. The following considerations must be made when selecting a vacuum advance spec:

How much engine vacuum is produced at cruise? If max vacuum at cruise, on a car with a radical cam, is only 15 inches Hg, a vacuum advance control unit that needs 18 inches to peg out would be a poor selection.

How much centrifugal advance (“total timing”) is in effect at cruise rpm? If the distributor has very stiff centrifugal advance springs in it that allow maximum timing to only come in near red-line rpm, the vacuum advance control unit can be allowed to pull in more advance without the risk of exceeding the 52-degree maximum limit. If the engine has an advance curve that allows a full 36-degree mechanical advance at cruise rpm, the vacuum advance unit can only be allowed to pull in 16 more degrees of advance.

Are you using “ported” or “manifold” vacuum to the distributor? “Ported” vacuum allows little or no vacuum to the distributor at idle. “Manifold” vacuum allows actual manifold vacuum to the distributor at all times.

Does your engine require additional timing advance at idle in order to idle properly? Radical cams will often require over 16 degrees of timing advance at idle in order to produce acceptable idle characteristics. If all of this initial advance is created by advancing the mechanical timing, the total mechanical advance may exceed the 36-degree limit by a significant margin. An appropriately selected vacuum advance unit, plugged into manifold vacuum, can provide the needed extra timing at idle to allow a fair idle, while maintaining maximum mechanical timing at 36. A tuning note on this: If you choose to run straight manifold vacuum to your vacuum advance in order to gain the additional timing advance at idle, you must select a vacuum advance control unit that pulls in all of the advance at a vacuum level 2” below (numerically less than) the manifold vacuum present at idle. If the vacuum advance control unit is not fully pulled in at idle, it will be somewhere in its mid-range, and it will fluctuate and vary the timing while the engine is idling. This will cause erratic timing with associated unstable idle rpm. A second tuning note on this: Advancing the timing at idle can assist in lowering engine temperatures. If you have an overheating problem at idle, and you have verified proper operation of your cooling system components, you can try running manifold vacuum to an appropriately selected vacuum advance unit as noted above. This will lower engine temps, but it will also increase hydrocarbon emissions on emission-controlled vehicles.

Thus, we see that there are many variables in the selection of an appropriate control unit. Yet, we should keep in mind that the control unit is somewhat of a “finesse” or “final tuning” aid to obtain a final, refined state of tune; we use it to just “tweak” the car a little bit to provide that last little bit of optimization for drivability and economy. The vacuum advance unit is not used for primary tuning, nor does it have an effect on power or performance at wide open throttle.

With these general (and a little bit vague, I know…) concepts in mind, let’s review a few concepts and terms. Then it’s on to the master listing of specs and parts…..:

Part Number
There are many different sources for these control units. Borg Warner, Echlin, Wells, and others all sell them in their own boxes and with their own part numbers. Actually, there are very few manufacturers of the actual units: Dana Engine Controls in Connecticut manufactures the units for all three of the brands just mentioned, so it doesn’t make much difference who you buy from: They’re made by the same manufacturer. The part numbers I have listed here are the NAPA/Echlin part numbers, simply because they are available in any part of the country.

ID#
Every vacuum advance control unit built by Dana, and sold under virtually any brand name (including GM), has a stamped ID number right on top of the mounting plate extension. This ID, cross referenced below, will give you all specifications for the unit. So now, when you’re shopping in a junkyard, you’ll be able to quickly identify the “good” vs. the “bad” control units.

Starts @ “Hg
Vacuum is measured in “inches of Mercury.” Mercury has the chemical symbol “Hg.” Thus, manifold vacuum is measured and referred to as “Hg. The “Start” spec for the control unit is a range of the minimum vacuum required to get the control unit to just barely start moving. When selecting this specification, consideration should be made to the amount of vacuum that a given engine produces, and what the load is on the engine at this specification. For example, an engine with a very radical cam may be under very light load at 7 inches Hg, and can tolerate a little vacuum advance at this load level. Your mom’s Caprice, on the other hand, has such a mild cam that you don’t want the vacuum to start coming in until 9 – 10 inches Hg. For most street driven vehicle performance applications, starting the vacuum advance at about 8” Hg produces good results.

Max Advance
Since the vacuum advance control unit is a part of the distributor, the number of degrees of vacuum advance is specified in DISTRIBUTOR degrees – NOT crankshaft degrees. When talking about these control units, it is important that you know whether the person you’re talking to is referring to the distributor degrees, or if he’s talking crankshaft degrees. All of the listings shown in the following chart, and in any shop manual & technical spec sheet, will refer to distributor degrees of vacuum advance. You must DOUBLE this number to obtain crankshaft degrees (which is what you “see” with your timing light). Thus, a vacuum advance control unit with 8 degrees of maximum advance produces 16 degrees of ignition advance in relationship to the crankshaft. When selecting a unit for max advance spec, the total centrifugal timing at cruise must be considered. Thus, a car set up to produce 36 degrees of total mechanical advance at 2500 rpm needs a vacuum advance control unit producing 16 degrees of crankshaft advance. This would be an 8-degree vacuum advance control unit.

Max Advance @ “Hg
This is the range of manifold vacuum at which the maximum vacuum advance is pegged out. In selecting this specification, you must consider the vacuum produced at cruise speed and light throttle application. If your engine never produces 20” Hg, you better not select a control unit requiring 21” Hg to work.

The following listing (Non-HEI) is as follows: The first two part number listings are the two numbers that are most commonly used in a Chevrolet performance application. The “B1” can is the most versatile and user-friendly unit for a good performance street engine. As you can see, it was selected by GM for use in most high performance engines due to its ideal specs. The “B28” can was used on fuel injected engines and a few select engines that produced very poor vacuum at idle. The advance comes in very quick on this unit – too quick for many performance engines. Do not use this very quick unit unless you have a cam/engine combination that really needs an advance like this. It can be used as a tuning aid for problem engines that do not respond well to other timing combinations, and can be successfully used in applications where direct manifold vacuum is applied to the can (see paragraph and discussion on this above)

After this, the listing is by Echlin part number. The Chevrolet applications are listed first by application, followed by a complete listing of all of the units used on any GM product (all GM units are interchangeable, so you can use a Cadillac or GMC Truck unit on your Vette, if that’s what you want to do).

Non-HEI Distributors:

P/N ID# Application Starts @ “Hg Max Adv
(Distr. Degrees @ “Hg.)

VC680 B1 1959 – 63 All Chevrolet 8-11 8 @ 16-18
1964 Corvette exc. FI
1964 Impala, Chevy II
1965 396 High Perf.
1965-67 283, 409
1966-68 327 exc. Powerglide
1967-68 All 396
1969 Corvette 427 High Perf.
1969 396 Exc. High Perf.
1969 Corvette 350 TI
1969-70 302 Camaro
1970 400 4-bbl
1970 396 High Perf.
1970 Corvette 350 High Perf.
1973-74 454 Exc. HEI

VC1810 B28 1965 409 High Perf. 3-5 8 @ 5.75-8
1965 327 High Perf.
1966 327 High Perf.
1964-67 Corvette High Perf. FI

------------------------- ------------------------- ------------------------- ------------------------- ------------------------- ---------------

VC1605 B9 1965 impala 396 Exc. High Perf. 7-9 10.3 @ 16-18
1965 327 All Exc. FI
1969 327 Camaro, Chevelle, Impala
1969-70 Corvette 350 Exc. High Perf.
1969-70 350 4-bbl Premium Fuel
1970 350 Camaro, Chevelle, Impala High Perf.
1971-72 350 2-bbl AT
1971-72 307 All

VC1675 B13 1968 327 Camaro Powerglide 9-11 8 @ 16-18
1968 327 Impala AT
1968 307 AT
1968 302, 307, 327, 350 Camaro, Chevy II
1970 350 Camaro, Chevelle Exc. High Perf.

VC1760 B19 1969 350 Camaro, Chevelle, Impala 4-bbl 5.5-8 12 @ 14-18
1969-70 350 2-bbl

VC1765 B20 1965 396 Impala High Perf 5-7 8 @ 11-13
1966-67 Corvette Exc. High Perf.
1966-67 Impala 427 Exc. High Perf.
1966-68 327 Powerglide Exc. High Perf.
1969 307 All
1969-70 396, 427 Camaro, Chevelle High Perf.
1970 400 2-bbl
1970 307 MT
1973 Camaro 350 High Perf.

VC1801 B21 1971 350 2-bbl 7-9 10 @ 16-18
1971-72 400, 402
1971-72 307 AT

VC1802 B22 1971-72 350 4-bbl 7-9 8 @ 14-16


Other Part Numbers & Specs:

VC700 B3 8-10 11.5 @ 19-21
VC1415 M1 6-8 10 @ 13-15
VC1420 M2 5-7 11 @ 16-17
VC1650 B12 8-10 10 @ 15-17
VC1725 B18 8-10 12 @ 13-16
VC1740 A5 6-8 12 @ 15-17.5
VC1755 A7 8-10 12.5 @ 18-20.5
VC1804 B24 6.5-8.5 10 @ 12-14
VC1805 M13 6-8 12 @ 14.5-15.5
VC1807 B25 5-7 8 @ 13-15
VC1808 B26 5-7 8 @ 11-13
VC1809 B27 5-7 9 @ 10-12
VC1812 B30 5-7 12 @ 11.75-14



The following listing (HEI) is as follows: The first four part number listings are the 4 numbers that are most commonly used in a Chevrolet performance application. The “AR12” can is the most versatile and user-friendly unit for a good performance street engine. The AR 15 and AR23 are almost identical, with only slight variations in their “start-stop” specs. The “AR31” can is the HEI equivalent to the “B28” Hi-Perf can used on the early engines: The advance comes in very quick on this unit – too quick for many performance engines. Do not use this very quick unit unless you have a cam/engine combination that really needs an advance like this. It can be used as a tuning aid for problem engines that do not respond well to other timing combinations, and can be successfully used in applications where direct manifold vacuum is applied to the can (see paragraph and discussion on this above)

After this, the listing is by Echlin part number. All GM HEI vacuum advance units are interchangeable, so you can use a Cadillac or GMC Truck unit on your Vette, if that’s what you want to do.

HEI Distributors:

P/N ID# Application Starts @ “Hg Max Adv
(Distr. Degrees @ “Hg.)

VC1838 AR12 1975 350 Buick 7-9 7 @ 10-12

VC1843 AR15 1977 305 All Exc. Hi Alt, Exc, Calif. 3-5 7.5 @ 9-11
1974 400 All w/2-bbl
1977 305 El Camino
1976 262 Monza Exc. Calif
1976 350 Vette Hi Perf, Incl. Calif
1975 350 Z-28
1977 305 Buick Skylark

VC1853 AR23 1976 350 All Calif. 5-7 7.5 @ 11-12.5
1976 350 Vette Calif., Exc. Hi Perf
1976 400 All, Exc. Calif
1975 350 4-bbl
1974 350 All w/1112528 Distr.
1978 350/400 Heavy Duty Truck, Exc. Calif, Exc. Hi Alt.

VC1862 AR31 2-4 8 @ 6-8




 
NYH1 
Contributor
Posts: 708

Loc: Central New York
Reg: 07-15-09
04-06-10 12:58 PM - Post#1893896    
    In response to steeler_fan

So would I be better off using an aftermarket adjustable vacuum advance set up like the Crane version or one of the factory cans listed above?

I built a mild 385 stroker motor (4.040" bore x 3.750" stroke). Compression ratio's are SCR 9.1 and DCR 7.93 with the cam I'm using. Which is a Lunatic Voodoo 262/268 Hydraulic Flat Tappet Cam, 219/227 dour. @.050", .468/.489 lift, 112 LSA 1400-5800 RPM range PT# 60102.

I'm using Summit Vortec Heads with a Edelbrock Vortec Performer RPM intake and for now a Holey 80457 600 CFM carb until I get an Edelbrock 650 AVS.

I'm going to use THIS MSD HEI ignition upgrade kit. I want a clean look and don't want to use an "MSD" type external ignition box. My distributor is in pretty good condition. I also have to get a spring kit to set up my machanical advance.

I've read that motors with Vortec heads only like 32° to 34° of total timing. Is that true?



'78 Camaro, mild Vortec head 385 stroker, 9.1 comp. Lunati Voodoo 262/268 hyd. ft. cam, RPM Intake, 650 AVS carb, TH350 Coan 11" 2600 stall, 8.5" 10 bolt. 3:42 posi, 1 5/8" Full length Headers, 2 1/2" Flowmaster Exhaust System.


 
Huneycutt 
Forum Newbie
Posts: 21
Huneycutt
Loc: North Carolina
Reg: 01-30-09
04-06-10 03:09 PM - Post#1893952    
    In response to NYH1

I recently did a lot of testing with an HEI ignition on the dyno, including with and without the vacuum advance. Admittedly, the vacuum advance is more for road manners than all out power, but if you are interested, there's a video with all the info here:

http://streetmuscleaction .com/engines/hei/

I think the vacuum advance testing is in the second one.

www.HorsepowerMonster.com


 
Jim_Elliott 
Senior Member
Posts: 250

Loc: La Quinta Calif
Reg: 11-06-01
04-06-10 03:58 PM - Post#1893969    
    In response to Mr. Sinister

  • Mr. Sinister Said:
yeah, switching to full manifold vacuum probably doesn't do much for a stock engine, it's definitely more for performance applications.



Nope...
On my motor home with a 454 it makes that critter sing right along from the initial startup, Just make sure you have corrected the vacuum advance..
My V.A. is now 11º with a 22º mechanical advance and 11º base timing (8.5 to 9.2 MPG and strong on the hill climba..

Jim




 
IgnitionMan 
Valued Contributor
Posts: 3193

Reg: 04-15-05
04-06-10 07:16 PM - Post#1894089    
    In response to Jim_Elliott

Howdy, Jim, glad to see you posting.

NOPE, full manifold vacuum advance, more correctly referred to what it really is, a LOAD COMPENSATION DEVICE, should be on full manifold vacuum, IF THE ENGINE DOES NOT HAVE A FUNCTIONING EGR VALVE. If the engine DOES have a functioning EGR valve, ported vacuum sourcing is just fine, as that emissions laden engine is not set up for any kind of performance, nor good stock operation. It's as simple as that.

And, for ANY and ALL vacuum advances, there should be a positive stop on the pull pin, so the load compensator will NOT over time the engine, NO MATTER HOW THE ENGINE IS SET UP, STOCK TO HOT ROD.



 
Petroholic 
"7th Year" Silver Supporting Member
Posts: 847
Petroholic
Age: 50
Loc: Moses Lake, Wa.
Reg: 08-12-05
04-06-10 07:40 PM - Post#1894113    
    In response to IgnitionMan

Anyone know where I can get one of those Crane cans?




 
Mr. Sinister 
Contributor
Posts: 554
Mr. Sinister
Age: 40
Loc: Fair Hill, MD
Reg: 05-18-09
04-07-10 04:00 AM - Post#1894227    
    In response to Petroholic

summit or jegs sells a few different adjustable cans.

Bill -
55 Chevy 210 2 Door Sedan Hot Rod
http://imgur.com/a/CfOc6
68 Caddy Coupe DeVille Kustom
http://imgur.com/a/m7gdR


 
C10 Sleeper 
Valued Contributor
Posts: 3426
C10 Sleeper
Loc: Redding Ca
Reg: 03-17-09
04-07-10 08:58 AM - Post#1894356    
    In response to Mr. Sinister

I bought a Crane VA at the local kragen auto parts store.

http://photobucket.com/C10Pictures


 
NYH1 
Contributor
Posts: 708

Loc: Central New York
Reg: 07-15-09
04-07-10 01:21 PM - Post#1894461    
    In response to C10 Sleeper

No one answered my question. Would I be better off using one of the V/A cans that steeler_fan posted, or an aftermarket can like the Crane can?

Thanks!



'78 Camaro, mild Vortec head 385 stroker, 9.1 comp. Lunati Voodoo 262/268 hyd. ft. cam, RPM Intake, 650 AVS carb, TH350 Coan 11" 2600 stall, 8.5" 10 bolt. 3:42 posi, 1 5/8" Full length Headers, 2 1/2" Flowmaster Exhaust System.


 




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